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stupid question

yes I know, everyone but me knows the answer, please forgive my ignorance, I'm looking to buy a couple of lenses for my 50d canon, I see my camera takes the two types, EFS and EF mount, what is the difference please and which is the better if either

While that link provided by Andrew is a good reference, there are a couple of things that I wouldn't agree with. In particular the first comment - Quote "The differences between the two lenses are purely cosmetic." That is just not right, and it says so itself by going on to describe the differences in the mount and focus position etc. (Edit, DO NOT try to mount a EF-S lens on a full frame camera like a 5D, you will damage the camera!)

It also says there is no difference in quality which is IMO far from true. While the EF-S lenses can produce some good results, the EF lenses (in which are "L" series) are better quality. That can be seen simply by the price (not always true in life, but you generally get what you pay for). Also there are no EF-S lenses worthy of the infamous Canon "L" label. It is sometimes said that the EF-S 10-22 is and "L" quality lens, but it certainly isn't named as such.

You will generally find that most of the EF-S range don't have large apertures, there are only a couple that go to F2.8, most are F3.5 or smaller. To get into the large aperture (faster lenses) you will need to go to an L series lens which are all EF mount. Also the L series will generally focus much faster and more accurately.

In summary, there is nothing wrong with buying an EF-S lens, they can be very good value for money, they are light weight, often cover very useful focal ranges. If it is quality and speed (both in terms of aperture and focus) you crave or you plan on going to a 5D or 1D in the future you should look at the EF mount or more specifically an "L" series lens, but these come at a cost!

The best site I have seen for lens reviews is this one: http://www.the-digital-picture.com/ It provides all the technical stuff if you are into that, but also provides real world practical examples that I find are easy to relate to.

Hey Mic, thanks for that, I will have a nose around that site but truthfully I want little more than basic knowledge, too much technical stuff leaves me snoring, I have the canon 50d and wish to mount the 10-22mm wide angle lens but have seen it either advertised as EF or EFS mount, I dont want to inadvertently try to mount the wrong lens, I also want the canon 100mm macro, bigger lenses will come later!, I intend to buy these lenses once only so quality and speed is of top priority

The 10-22 is definitely a EF-S mount. You can't go wrong with the 50D though, it will take either EF or EF-S. You'll find though as quality and speed are your priorities you'll end up going for a "L" series lens.

Be warned though, once you buy one you will catch "L fever" and won't be able to stop!